1. Field of the Invention
The present invention relates to a layered low-pass filter formed using a layered substrate.
2. Description of the Related Art
Recently, there have been strong demands for reductions in size and thickness of electronic apparatuses for portable use typified by cellular phones and notebook personal computers, and accordingly, reductions in size and thickness have also been required for electronic components for use in such electronic apparatuses. Low-pass filters are one of the electronic components for use in such electronic apparatuses. Reductions in size and thickness are also required for low-pass filters. To meet this, it has been proposed to form a low-pass filter using a layered substrate, as disclosed in, for example, JP-A-5-259703, JP-A-5-299962, JP-A-6-097701 and JP-A-2000-101378. A low-pass filter formed using a layered substrate as mentioned above will be hereinafter called a layered low-pass filter.
To obtain a steep attenuation characteristic with a low-pass filter, the low-pass filter should preferably be of the fifth order or higher. A fifth-order low-pass filter includes, for example, a first inductor and a second inductor connected in series to each other, a first capacitor provided between the ground and the node between the two inductors, a second capacitor provided between the ground and an end of the first inductor opposite to the node, and a third capacitor provided between the ground and an end of the second inductor opposite to the node.
Inverse Chebyshev low-pass filters and elliptic (simultaneous Chebyshev) low-pass filters are known as being capable of providing a steeper attenuation characteristic. The inverse Chebyshev low-pass filters and the elliptic low-pass filters present at least one attenuation pole in the stop band.
For a signal path in which a low-pass filter is used, there are cases where it is required to increase attenuation especially in a specific narrow frequency band present in the stop band of the low-pass filter. An example of such cases is a case where, in a communication apparatus operable on a first communication system that uses a first frequency band and a second communication system that uses a second frequency band higher than the first frequency band, a low-pass filter for allowing signals in the first frequency band to pass and intercepting signals in the second frequency band is used in the path of the reception signal of the first communication system. In such a case, in the reception signal path in which the low-pass filter is used, it may be required to increase attenuation especially in the second frequency band present in the stop band of the low-pass filter.
In the case where it is required to increase attenuation especially in a specific narrow frequency band present in the stop band of a low-pass filter in the signal path in which the low-pass filter is used as described above, a notch filter for increasing attenuation in the specific narrow frequency band may be provided in series to the low-pass filter. Disadvantageously, however, this leads to an increase in insertion loss in the pass band of the low-pass filter.
JP-A-5-259703 discloses a technique of adjusting the frequency of the attenuation pole closest to the pass band by adjusting the inductance generated in an external electrode portion through changing the length of the external electrode. However, in the case where it is required to increase attenuation especially in a specific narrow frequency band present in the stop band of a low-pass filter in the signal path in which the low-pass filter is used, the specific narrow frequency band is typically present within a range from the frequency at which the attenuation pole closest to the pass band appears to a frequency on the order of several times higher than the cut-off frequency. In such a case, it is difficult to increase attenuation especially in the specific narrow frequency band present in the stop band by utilizing the attenuation pole closest to the pass band.